Additive manufacturing systems are used to build three-dimensional (3D) parts from digital representations of the 3D parts using one or more additive manufacturing techniques. Common forms of such digital representations would include the well-known AMF and STL file formats. Examples of commercially available additive manufacturing techniques include extrusion-based techniques, ink jetting, selective laser sintering, powder/binder jetting, electron-beam melting, and stereolithographic processes. For each of these techniques, the digital representation of the 3D part is initially sliced into a plurality of horizontal layers. For each sliced layer, a tool path is then generated, that provides instructions for the particular additive manufacturing system to form the given layer.
For example, in an extrusion-based additive manufacturing system, a 3D part (sometimes referred to as a 3D model) can be printed from the digital representation of the 3D part in a layer-by-layer manner by extruding a flowable part material. The part material is extruded through an extrusion tip carried by a printhead of the system, and is deposited as a sequence of layers on a substrate in an x-y plane. The extruded part material fuses to previously deposited part material, and solidifies upon a drop in temperature. The position of the printhead relative to the substrate is then incremented along a z-axis (perpendicular to the x-y plane), and the process is then repeated to form a 3D part resembling the digital representation.
In fabricating 3D parts by depositing layers of a part material, supporting layers or structures are typically built underneath overhanging portions or in cavities of objects under construction, which are not supported by the part material itself. A support structure may be built utilizing the same deposition techniques by which the part material is deposited. The host computer generates additional geometry defining the support structure for the overhanging or free-space segments of the 3D part being formed, and in some cases, for the sidewalls of the 3D part being formed. The support material adheres to the part material during fabrication, and is removable from the completed 3D part when the printing process is complete.
In two-dimensional (2D) printing, electrophotography (also known as xerography) is a technology for creating 2D images on planar substrates, such as printing paper and transparent substrates. Electrophotography systems typically include a conductive support drum coated with a photoconductive material layer, where latent electrostatic images are formed by electrostatic charging, followed by image-wise exposure of the photoconductive layer by an optical source. The latent electrostatic images are then moved to a developing station where part material is applied to charged areas, or alternatively to discharged areas of the photoconductive insulator to form visible images. The formed part material images are then transferred to substrates (e.g., printing paper) and affixed to the substrates with heat and/or pressure.
U.S. Pat. No. 9,144,940 (Martin), entitled “Method for printing 3D parts and support structures with electrophotography-based additive manufacturing,” describes an electrophotography-based additive manufacturing method that is able to make a 3D part using a support material and a part material. The support material compositionally includes a first charge control agent and a first copolymer having aromatic groups, (meth)acrylate-based ester groups, carboxylic acid groups, and anhydride groups. The part material compositionally includes a second charge control agent, and a second copolymer having acrylonitrile units, butadiene units, and aromatic units.
The method described by Martin includes developing a support layer of the support structure from the support material with a first electrophotography engine, and transferring the developed support layer from the first electrophotography engine to a transfer medium. The method further includes developing a part layer of the 3D part from the part material with a second electrophotography engine, and transferring the developed part layer from the second electrophotography engine to the transfer medium. The developed part and support layers are then moved to a layer transfusion assembly with the transfer medium, where they are transfused together to previously-printed layers.
One issue that can arise in electrophotographic printing is that even small registration errors between the layers of part material can introduce non-uniformities in what should be smooth vertical surfaces that are easily detectible, both visually and tactilely. Registration errors in an electrophotographic printing system are typically on the order of 100 microns, even using standard registration compensation methods. This is insufficient to eliminate the detectable surface non-uniformities. There remains a need for an improved method for printing a three-dimensional part with an electrophotography-based additive manufacturing system to improve the registration of the part layers.